[clang] [SYCL] Basic diagnostics for the sycl_kernel_entry_point attribute. (PR #120327)

[Tom Honermann via cfe-commits]

tahonermann wrote:

@erichkeane, thank you for the review comments so far. I think I've addressed or responded to all of them and I look forward to your next round of review. I don't expect you to see this message until January 6th or so, so I hope you are enjoying, or have enjoyed, the holidays!

https://github.com/llvm/llvm-project/pull/120327